The only way to get the result America wants in negotiations with North Korea is to get a nuclear force in place in South Korea and trade it away in exchange for the North's nuclear arsenal. The United States made a terrible mistake to pull its nuclear weapons out of South Korea in 1991, but today it has a chance to rectify that error. By now North Korea has made it clear that it is demanding more than money to give up its nuclear program—it wants a strategic trade-off. Such demands are what led to the summit getting temporarily called off in the first place. North Korea reiterated its own definition of "denuclearization", in which America must withdraw its troops and its nuclear umbrella over the South in return for the North giving up its nuclear weapons.

But America can't afford to do that. It would verge on deeding South Korea over to the North and would further endanger Japan and the peace of the region. Like most unrealistic things proposed in the name of peace, it would be a disaster for the peace of the world.

The drastic fluctuations in the diplomacy of recent weeks have made apparent an underlying weakness in the U.S. negotiating position. America's distant and extended deterrence posture has left Washington without spare strategic nuclear chips on the ground to trade off in return for the strategic nuclear chips from the other side. The U.S. needs to create spare strategic chips that can be bargained away. America is late to realize this but better late than never. Otherwise, if America fails to do this, the negotiations are doomed.

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U.S. nuclear weapons should be in South Korea—and Japan as well. Then North Korea will finally have reason to negotiate away its nuclear forces.

Washington could simply put its some of its tactical nuclear weapons back in South Korea as it had stationed there before. But the U.S. could also do better by loaning such weapons to its allies as a semi-independent nuclear deterrent.

That would change the facts on the ground dramatically by turning the strategic calculations of China and North Korea upside down. They would no longer be able to assume that they will end up dominating the region by using North Korea's nuclear program to blackmail America. Instead, North Korea and China would find themselves on the worse end of the atomic stick.

Suddenly it would become a vital strategic interest on their part to bargain away the North's nuclear capabilities for the South's. Bargaining would proceed from the vantage point of that changed reality.

South Korea and Japan would become, for the moment, de facto independent nuclear powers. As a result, North Korea and China would be staring their worst nightmare in the face—the prospect that, if they didn't bargain wholesale, the South and Japan would become permanently independent nuclear powers.

That would get their attention. America would finally have something big enough to trade for North Korea's nuclear weapons. It would be something that North Korea and China would be desperately anxious for the U.S. to bargain over, no longer something to play coy about.

It is a method President Donald Trump can be expected to appreciate. After all, as a candidate, Trump said that South Korea and Japan might have to go nuclear.

Trump's point was dismissed cavalierly by the media as “unacceptable”. But then, Trump himself was also dismissed as “unacceptable”, and impossible to boot. Now he is in the White House.

Furthermore, South Koreans themselves think, by a substantial majority—60 percent, in repeated polls—that they should have their own nuclear weapons. It is time for America's media to stop imagining itself a kind of moral super-government that can declare vital policy options "unacceptable", and to instead start respecting sober common sense. The South Koreans, when they favor a nuclear capacity, are merely facing up to the reality of an existential threat that they do not have the luxury to ignore. As North Korea's nuclear range extends into the U.S., America too should realize that the U.S. no longer has the luxury to ignore this reality.

In other words, Trump was right, and his suggestion was elementary realism. The point now is to work out the details and overcome the danger of proliferation, not talk up the North Korean threat as if it were insuperable. This means the U.S. should loan a nuclear force to South Korea and Japan, with America retaining final ownership.

The loan could be structured as follows:

1. The force should be sizable and considerably larger than North Korea's.

Japan and South Korea should each get a force and, for survivability, it should consist of sea-based submarines carrying nuclear missiles.

An optimal force size and shape has been worked out in a TNI blog, which suggests “a force of five ballistic missile submarines, each equipped with sixteen nuclear-tipped missiles. Each missile would be equipped with four 100 kiloton warheads. The one submarine on patrol at all times would be equipped with sixty-four warheads.”

Finally, these forces should be loaned, not self-developed, for three critical reasons: speed, non-proliferation, and avoiding an unstable transitional period while they are provided to America’s allies.

2. Each ally should have autonomous power to launch its missiles.

It should not be a traditional dual-key arrangement, which is still a form of extended U.S. deterrence. Extended deterrence puts the U.S. in the line of fire for nuclear retaliation, and leaves uncertainty about whether the U.S. would really launch the forces to protect an ally. It's safer for America if its allies have an operationally independent deterrent, with America not responsible for their use—and this is safer for South Korea and Japan too.

The U.S. nuclear arsenal would be kept entirely separate and would remain reserved for its sole purpose: to deter any attack on the U.S.

3. In return for granting autonomous use, the U.S. should maintain ultimate ownership of those nuclear weapons, and restrict what countries the missiles can target.

This is what keeps this nuclear weapons loan program from becoming a form of proliferation that could worry other regional countries or cause them to go nuclear in response.

In addition, to bolster this ownership and prevent any unauthorized nationalization of those weapons, the U.S. should provide all technical maintenance and keep essential technological secrets undisclosed. Thus there would be no unilateral nationalization of the forces and no unilateral withdrawal of them either, except on pre-defined terms.

4. America should pre-define the terms for repossessing or altering these forces. Here are some possible terms:

America will take the South Korean nuclear loan back only if North Korea gives up all its nuclear forces and missiles. Also, the U.S. will loan more nuclear weapons to South Korea if North Korea doesn't disarm, or if it resumes its tests and nuclear buildup.

Washington will take back the Japanese nuclear loan if China and Russia reach a diplomatic accommodation with Japan on maritime, territorial, and nuclear issues.

Finally, if negotiations go nowhere too long, America retains the option of transferring permanent ownership outright to its allies. This conveys a clear message: the U.S. is ready to deal, but the loan is not a bluff.

North Korea would finally realize that, if it doesn't want to face a nuclear South Korea forever--and one that will always be stronger than it is--it will have to negotiate away its forces.

China similarly would realize that, if it doesn't want to face a strong nuclear Japan forever, it will not only have to force North Korea to give up its nuclear programs; it will also have to reach an accommodation with Japan on strategic issues. This could have profoundly beneficial effects for America. In the absence of a South Korean or Japanese nuclear weapons program, the U.S. has been wishing for nuclear arms control via China for decades but getting next to nowhere on it. The loan would change immediately the balance in all of these matters.

Washington would become the party in the stronger bargaining position and would no longer be pleading. Instead, China and North Korea would need to start pleading with the U.S. and its allies. Then everyone would have to bargain for real.

Thus far, the Chinese and Russians have preferred to keep the North Korean nuclear threat intact, as a way of extracting concessions from America. They have followed a good cop bad cop routine, making a show of helping the U.S. against the bad cop while in practice still playing primarily on the other side. It has served them well.

Economic sanctions have not changed China's basic calculation on this. It continues playing North Korea against America, as best it still can. It recently had to make concessions to Trump and put more economic pressure on North Korea, but its ally Russia partly filled in for it in supplying the North. After South Korea got North Korea to agree to the summit, Kim went straight to China, which encouraged him to stiffen his posture and only react halfway. Therefore, it will take the shock of a loaned nuclear force, upending China's entire regional calculus, to get it to reverse its policy of favoring North Korean nuclear weapons for its own strategic purposes.